* when appending log field only do so by minimal amount
* move slice utils to separate package to fix import cycle, add GrowJust() and AppendJust() functions
* fix GrowJust() not returning slice of same length
* improved xslices tests
* make AppendJust() test check for slice contents, fix AppendJust() final copying behaviour
* add a +1 with field growth to try minimise allocation for log 'msg' field
